1. Field of the Invention
The invention relates to compositions useful for extrusion coating wire and cables. More specifically, the compositions of the invention which exhibit improved melt flow under high shear comprise a blend of a major proportion of a first polymeric component which is a mixture of crystalline polypropylene homopolymer and amorphous or semi-crystalline ethylene-propylene copolymer and a minor proportion of a second polymer component which is a fractional melt index ethylene polymer, said blend dynamically modified in the presence of an organic peroxide.
2. Description of the Prior Art
Polyolefin resins are widely utilized for the construction of insulated wire and cable products. For high-speed extrusion of wire and cable insulation, the insulation composition should have a melt flow rate of about 1 to 5 g/10 min and, more preferably, 1.5 to 3.5 g/10 min for best results. Whereas polypropylene (PP) resins generally have better dielectric properties and abrasion resistance than polyethylene (PE) resins, processability problems have limited its use for extrusion coating of wire and cable products.
Particularly troublesome is the phenomenon called melt fracture. This phenomenon can be observed with virtually all polyolefin resins, including polyethylenes such as linear low density polyethylene (LLDPE) and high density polyethylene (HDPE); however, it is more pronounced and less easily controlled with PP resins. Melt fracture results in the distortion observed with extrudates obtained when resins are extruded through annular dies and capillaries at flow rates above certain critical limits. At first this appears as surface waviness or roughness but, as flow rates continue to increase, it results in the formation of helical extrudates and, ultimately, gross melt fracture.
Severe melt fracture during extrusion coating of electrical conductors results in non-uniform thickness of the insulation around the conductor, i.e., eccentricity. Failure to have the conductor consistently positioned at the geometric center of the construction, i.e., concentric, can result in decreased signal performance and crosstalk. Furthermore, those areas where thickness of the insulation layer is inadequate are more prone to pinholes and cracking from bending or abrasion. As wire manufacturers continue to push for higher line speeds, eccentricity becomes one of the major limiting factors. Melt fracture and its effect on conductor eccentricity is described in an article by J. S. Borke entitled, "Oscillatory Flow of Polypropylene and Its Effect on Conductor Eccentricity," Proceedings of the International Wire and Cable Symposium, 47, 294-298 (1998).
It is highly desirable to have PP compositions within the desired melt flow range suitable for insulating wires and cables which exhibit improved melt flow during extrusion coating, thus minimizing eccentricity within the finished insulated article. It is more advantageous if the compositions are capable of being utilized at high line speeds for the production of telecommunication singles.
U.S. Pat. No. 4,375,531 to Ross discloses high impact visbroken, either by chemical or thermal treatment, blends of propylene and ethylene polymer components. Even though in one embodiment Ross discloses the compositions can be reactor-made or physically blended mixtures of PP and random copolymer of propylene and ethylene with an ethylene polymer, such as HDPE, the reference does not disclose use of the visbroken blends for wire and cable extrusion coating. More importantly, Ross does not disclose or even remotely suggest that, by judicious selection of the polymer components, high melt flow rate compositions with significantly improved melt flow suitable for high-speed wire and cable extrusion can be obtained.